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Ocean Color

MODIS - Bahamas
This MODIS image of blue water in the Caribbean Sea looks blue because the sunlight is scattered by the water molecules. Near the Bahama Islands, the lighter aqua colors are shallow water where the sunlight is reflecting off of the sand and reefs near the surface.

If someone were to ask you what is the color of the ocean, chances are that you would answer that is was blue. For most of the world's oceans, your answer would be correct. Pure water is perfectly clear, of course -- but if there is a lot of water, and the water is very deep so that there are no reflections off the sea floor, the water appears as a very dark navy blue. The reason the ocean is blue is due to the absorption and scattering of light. The blue wavelengths of light are scattered, similar to the scattering of blue light in the sky but absorption is a much larger factor than scattering for the clear ocean water. In water, absorption is strong in the red and weak in the blue, thus red light is absorbed quickly in the ocean leaving blue. Almost all sunlight that enters the ocean is absorbed, except very close to the coast. The red, yellow, and green wavelengths of sunlight are absorbed by water molecules in the ocean. When sunlight hits the ocean, some of the light is reflected back directly but most of it penetrates the ocean surface and interacts with the water molecules that it encounters. The red, orange, yellow, and green wavelengths of light are absorbed so that the remaining light we see is composed of the shorter wavelength blues and violets.

If there are any particles suspended in the water, they will increase the scattering of light. In coastal areas, runoff from rivers, resuspension of sand and silt from the bottom by tides, waves and storms and a number of other substances can change the color of the near-shore waters. Some types of particles (in particular, the cells of phytoplankton, also referred to as algae) can also contain substances that absorb certain wavelengths of light, which alters its characteristics.

Phytoplankton
Left Image: Phytoplankton are very small, single-celled plants, generally smaller than the size of a pinhead that contain a green pigment called chlorophyll. All plants (on land and in the ocean) use chlorophyll to capture energy from the sun and through the process known as photosynthesis convert water and carbon dioxide into new plant material and oxygen.

Right Image: Although microscopic, phytoplankton can bloom in such large numbers that they can change the color of the ocean to such a degree that we can measure that change from space. The swirls of green are a phytoplankton bloom in the Gulf of California.

The most important light-absorbing substance in the oceans is chlorophyll, which phytoplankton use to produce carbon by photosynthesis. Due to this green pigment - chlorophyll - phytoplankton preferentially absorb the red and blue portions of the light spectrum (for photosynthesis) and reflect green light. So, the ocean over regions with high concentrations of phytoplankton will appear as certain shades, from blue-green to green, depending upon the type and density of the phytoplankton population there. The basic principle behind the remote sensing of ocean color from space is this: the more phytoplankton in the water, the greener it is....the less phytoplankton, the bluer it is. There are other substances that may be found dissolved in the water that can also absorb light. Since these substances are usually composed of organic carbon, researchers generally refer to these substances as colored dissolved organic matter, CDOM for short.

The study of ocean color helps scientists gain a better understanding of phytoplankton and their impact on the Earth system. These small organisms can affect a system on a very large scale such as climate change. Phytoplankton use carbon dioxide for photosynthesis and in turn provide almost half the oxygen we breathe. The larger the world's phytoplankton population, the more carbon dioxide gets pulled from the atmosphere, hence, the lower the average temperature due to lower volumes of this greenhouse gas. Scientists have found that a given population of phytoplankton can double its numbers about once per day. In other words, phytoplankton respond very rapidly to changes in their environment. Large populations of these organisms, sustained over long periods of time, could significantly lower atmospheric carbon dioxide levels and, in turn, lower average temperatures. Carbon can be 'stored' in oceanic sediments when organic matter sinks and is buried in the ocean floor.

Understanding and monitoring phytoplankton can help scientists study and predict environmental change. Since phytoplankton depend upon sunlight, water, and nutrients to survive, physical or chemical variance in any of these ingredients over time for a given region will affect the phytoplankton concentrations. Phytoplankton populations grow or diminish rapidly in response to changes in its environment. Changes in the trends for a given phytoplankton population, such as its density, distribution, and rate of population growth or diminishment, will alert Earth scientists that environmental conditions are changing there. Then, by comparing these phytoplankton trends to other measurements - such as temperature - scientists can learn more about how phytoplankton may be contributing to, and affected by, climatic and environmental change.

SeaWiFS - Patagonian Shelf Bloom
Changes in ocean color can be caused by a variety of sources. In this SeaWiFS image, two currents are flowing past each other. The warm Brazil Current flows south next to the shore of Argentina. The colder Malvinas / Falkland current flows north, nearly parallel to the Brazil Current. (The Falkland Islands can be seen at the bottom of the image.) The interaction of these two currents brings nutrient-rich water from the deep ocean to the surface, providing an excellent environment for the growth of phytoplankton. The distinct populations of phytoplankton in each current can be perceived as different colors.

Below are several color samples extracted from this image, with a brief explanation of the likely cause of the dominant color.

 

SeaWiFS - Patagonian Shelf Bloom 1
SeaWiFS - Patagonian Shelf Bloom 2
The turquoise swirls of the Malvinas Current are likely colored by a bloom of coccolithophorids. In the first image, a tendril of dark-green Brazil Current water is mixing with the lighter blue of the Malvinas Current, and in the second image, clearer water from the adjacent Atlantic Ocean is mixing with the Malvinas Current. Coccolithophorids are phytoplankton that make microscopic spheres composed of calcium carbonate plates (called coccoliths). The bright white calcium carbonate spheres excel at reflecting light, producing the milky turquoise-blue color of a coccolithophorid bloom.
SeaWiFS - Patagonian Shelf Bloom 3 As the coccolithophorids are dispersed and sink in deeper, clearer ocean water, the color of the water deepens to azure.
SeaWiFS - Patagonian Shelf Bloom 4 This portion of the image features the light-brown, silt-laden water in the wide and shallow Rio de la Plata estuary, which lies just south of the city of Buenos Aires. The Rio de la Plata receives the flow of the Uruguay and Parana rivers, heavy with sediments eroded from the Andes Mountains. As the estuary deepens into the Atlantic, the light-brown color darkens to a greenish-brown.
SeaWiFS - Patagonian Shelf Bloom 5 This portion of the image shows the Bahia Blanca estuary, which receives the flow of several small rivers draining the southern Argentinian Pampas. Because these rivers carry less sediment than the larger Uruguay and Parana rivers, Bahia Blanca has clearer water, and the brown sediment color inland transforms to light green along the coast. In this case, the color seen from space may also be influenced by the reflection of light from the shallow sea floor.
SeaWiFS - Patagonian Shelf Bloom 6 East of the turquoise Malvinas current, the dispersed coccolithophorids in the current mix with the dark blue-black waters of the southern Atlantic Ocean.
SeaWiFS - Patagonian Shelf Bloom 7 This isolated patch of phytoplankton appears greenish-white, due to a combination of light reflection and light absorption.
SeaWiFS - Patagonian Shelf Bloom 8 This spiraling blue-green section of the Brazil Current is evidently populated by a different species of phytoplankton than the bright coccolithophorids floating just to the east, as indicated by its distinctly different color. Though this is clearly a phytoplankton bloom, the green color of chlorophyll does not vary much between the multitude of phytoplankton species found in the ocean. Therefore, even though ocean color data is used to estimate the concentration of chlorophyll in ocean waters, only under special circumstances can the actual identity of the phytoplankton in the bloom be determined solely from satellite data. However, scientists on ships can use remote-sensing data to find blooms and identify the species of phytoplankton in a bloom. (There is a particular kind of phytoplankton called blue-green algae, but even though this bloom is a shade of bluish-green, that doesn't mean that it's composed of this type of phytoplankton.)

Credit for all Patagonian images: Image courtesy Norman Kuring, SeaWiFS Project, image descriptions courtesy of James Acker, NASA GES DISC Oceans Data Team/SSAI.